The invention disclosed and claimed herein generally pertains to the field of apparatus and techniques for monitoring very low levels of light emission or radiation in an underwater environment. More particularly, the invention pertains to monitoring the emission of light in a natural body of water, such as a quantity of seawater, which is included in a particular range of wavelengths such as the ultraviolet range.
In monitoring broadband low level light radiation in an underwater environment, and particularly in a natural environment such as a body of seawater, light photons having wavelengths of less than 400 nm. have proven to be of great interest. Above 400 nm., bioluminescent light, i.e., light which is emitted by microorganisms present in an underwater environment, tends to be so dominant that it is impossible to determine whether light present in the environment has been generated by the organisms, or has been generated by some other broadband low level light source. Consequently, the present or absence of low level light sources other than bioluminescent organisms in a natural underwater environment may not be determined by monitoring light of a wavelength which is in excess of 400 nm.
In order to eliminate interferring effects of bioluminescent light in monitoring broadband low level light emission in an underwater environment, a selected optical filter may be employed with a photodetector. For example, if a photomultiplier tube is used for a detector, a filter may be interposed between the photo tube and the environment to prevent passage of any light of wavelength in excess of 400 nm. The filter therefore prevents most bioluminescent light from reaching the photodetector.
Visible light of less than 400 nm. wavelength is generally considered to be in the ultraviolet range or regime. It has been found that light in the 300-400 nm. range has an attenuation length of 5 meters in seawater, attenuation length being the distance that a light photon is likely to travel through a medium before being absorbed thereby. A 5 meter length is sufficient to enable significant numbers of ultraviolet light photons generated in proximity to a photodetector to pass through an optical filter and a protective window, or viewing port, at the photodetector before being absorbed. Photons of 300-400 nm. wavelength may therefore usefully be monitored, for example, to observe the activity of a low level light source.
A source of low intensity light emission may also generate light photons in the wavelength range 200-300 nm., and the detection of such photons may likewise be useful for monitoring the activity of a low level source. However, the attenuation length of photons of less than 300 nm. wavelength is less than 5 meters, and decreases rapidly down the range, so that the attenuation length of photons of 200 nm. wavelength is on the order of 1 centimeter. Consequently, very few photons in the 200-300 nm. wavelength range will be able to reach the photodetector, and pass through the optics thereof to be detected thereby. In addition, if monitoring activity requires a photodetector to remain in an underwater environment for an extended period of time, the window or viewing port of the detector must be coated with an antifouling material, to prevent biological growth thereupon. It has been found, for example, that after seven days, a viewing port placed in a body of natural seawater becomes so covered with biological growth that only 20% of the light can pass therethrough. However, substances commonly used to prevent biological fouling do not transmit light which is less than 300 nm. in wavelength.
The invention of Applicants overcomes the above problems by providing means for shifting light of 200-300 nm. wavelength into a wavelength range of 300-400 nm. Light in the 200-300 nm. range is thereby made detectable by a system which may be deployed in a natural underwater environment for a period of as long as three months, to monitor low levels of light occurring therein. By providing the capability to detect light in the 200-300 nm. range as well as in the 300-400 nm. range, substantial improvements in detection sensitivity may be realized.